This invention relates to electrographic recording systems. Voltages are impressed on record electrodes or styli as desired and an electrographic recording media in the proximity of the electrode is electrostatically charged to form a latent image. Toner is subsequently applied to the record medium to develop the image. The improvement described herein relates to circuitry for impressing a voltage on the record electrodes.
An electrographic writing system may have thousands of styli which must have a voltage impressed thereon quickly, effectively, and economically. Many different types of electrographic recording systems have been devised. For example, U.S. Pat. No. 3,662,396 incorporated herein by reference, discloses an electrographic recording system of the type utilizing coated paper as the record medium. A first array of recording electrodes is mounted in close proximity to the exposed surface of the charge retentive layer of the record medium. A second array of complementary electrodes is mounted adjacent to the first array in cooperative relationship in the record medium. A circuit is used to apply a first voltage of one polarity to the first array and a second voltage of opposite polarity to a second array of complementary electrodes. The second voltage thereby causes the potential of the conductive layer to change in coincidence with the first voltage.
Dual electrode systems have a relatively short write time. A single system is desirable to increase the time available for writing, reduce size, complexity and cost. Individual electrical drives for each stylus would impress voltages on the styli quickly, but are economically unfeasible for the large number of styli involved. Prior art devices have suggested the use of a plurality of high voltage switching transistors for each stylus. The quality and expense demanded by a plurality of switching transistors for each stylus also makes the possiblity undesirable.
One method of alleviating the above mentioned problems is disclosed in U.S. Pat. No. 4,115,763 and U.S. Pat. No. 4,058,814, both to Brown et al and assigned to the same assignee as the present invention and are incorporated herein by reference. The Brown et al patents show a switching system that uses a first high stored charge or slow diode and a second low stored charge or fast diode in series with a capacitor. When a stylus is desired to be powered the slow diode is set up with an initial forward biasing. A large negative voltage is then applied to the anode side of the slow diode in order to reverse bias it. Reverse current is then drawn through the slow diode for a short period of time which is sufficiently large to power the stylus for writing purposes.
A need has been felt to develop a different switching system using slow and fast diodes than what was taught in the aforementioned Brown et al patents.